Representatives from nearly 200 countries kicked off a key climate change summit on Monday in Paris, attempting to work together to curtail global warming and reduce carbon emissions.

The leaders are at the COP21 conference to try and keep global temperatures from rising more than 2 degrees Celsius from the Industrial Revolution, when humans began burning fossil fuels.

Climate change scientists have warned eclipsing that mark could be catastrophic for the planet.

According to the conference’s website, this two-week summit is the first time in more than 20 years of talks that leaders are trying to agree to legally binding, universal rules on the subject.

Two degrees Celsius is equal to 3.6 degrees Fahrenheit. According to CNN, global temperatures have already risen .85 degrees Celsius (or 1.53 degrees Fahrenheit) since the surge in carbon emissions, and it is expected that the feared 2-degree mark should be reached in this century.

CBS News reported some estimates have the global temperatures rising 3 to 5 degrees by 2100. Even just a 2 degree increase might lead to droughts, rising sea levels and mass extinctions.

With that known, representatives from the 195 countries at COP21 aren’t wasting any time.

President Barack Obama sat down one-on-one with Chinese President Xi Jinping, CNN reported. China and the United States are ranked 1-2 in global greenhouse gas emissions.

Reuters reported Obama said the two countries “have both determined that it is our responsibility to take action,” and would cooperate to develop a lasting agreement.

One of the biggest obstacles might be getting the nations to all agree on one set of rules, particularly when the countries represented range from mega powers to developing nations.

Some island nations are calling for a more strict goal of 1.5 degrees Celsius, the BBC reported. They, and other countries with low elevations, would obviously be impacted by rising sea levels.

There’s also the issue of how to pay for climate reform. The Wall Street Journal reported that some leaders have cautioned they won’t back a deal that doesn’t provide sufficient financing.

Fairness of the treaty is also expected to be a key issue, the BBC reported. Some nations have output more greenhouse gasses than others, and now they must cooperate to address the issue.

But the world is watching the leaders and hoping they come up with a concrete agreement.

The BBC reported that 570,000 protesters around the globe marched to support ending the rising temperatures, including 50,000 in London and a record number of 45,000 in Sydney.

The conference comes just days after Nov. 13 terrorist attacks killed 130 people in Paris. CNN reported the delegates observed a moment of silence for the victims before getting to work.